The invention relates generally to electromagnetic signalling and telemetry for biomedical implantable devices.
The increasing versatility of implanted stimulators such as cardiac pacers demands more complex programming capabilities. Programming in this context means noninvasively transferring parameter value data from an external device called the programmer to an internal device implanted in the patient's body. Outbound telemetry has been used in the past primarily to verify the programmed pulse parameters of implanted stimulators.
A number of programming systems have been successfully employed in commercially available cardiac pacers, including magnetic programming and radio frequency (RF) programming. Magnetic programming relies on the generation of a series of strong magnetic impulses which actuate a reed switch inside the pacer. The output of the reed switch circuit forms the programming input to data registers in the implant as shown, for example, in U.S. Pat. No. 3,805,796 to Terry et al, assigned to the assignee of the present application. In pacers such as the Cordis "Omnicor.RTM." series, the number of reed switch closures in sequence from 1 to approximately 135 is counted. The resulting count corresponds to a selected value of a specific parameter. Reed switches have a number of desirable attributes. Besides having little or no associated current drain in the quiescent mode, the insensitivity of reed switches protects against spurious programming. In addition, of course, it is possible to use the same sensor for the conventional diagnostic mode accessed by placing a permanent magnet over the pacer site. On the other hand, reed switch rates are limited to several hundred hertz, and, as pointed out in copending application Ser. No. 297,746, their physical size and sensitivity to orientation and proximity of the magnetic field are limitations. Moreover, electromagnetic programming requires a heavy coil and large current drain which increases the size and weight of the self-contained hand-held programming unit.
Reflected signal telemetry, introduced in copending application Ser. No. 153,093, has proved to be a significant advance in the art of implant telemetry. In this system, a 16 kHz carrier is externally transmitted and picked up and reflected by a coil in the implant capacitively tuned to the same frequency. A data-drive FET switch intermittently shunts the tuned coil thus modulating its reflectance characteristics. The data is recovered externally by pickup coils which feed the reflected signal through a relative phase shift detection system. The only power needed to operate the FET switch is supplied by the tuned coil transponder. 16 kHz was chosen because the customary titanium enclosure or "can" for present day pacers is relatively transparent to this frequency. In fact, the roll off above 20 kHz is at the rate of about 3 db per 30 kHz.
The amount of data flowing back and forth between the external programmer and the biomedical implant is on the increase. Programming requirements place ever greater strains on outbound as well as inbound telemetry. Outbound telemetry of intracardiac ECG's, for example, is a special challenge for low power implanted systems.